Admiral Mike Mullen, speaking on Saturday after meeting top military officials in Pakistan, also underscored the limitations of US action in dealing with the problem.

He conferred yesterday in Peshawar with Pakistani military commanders who are fighting the battle against extremists, a day after a suicide bomber in north-western Pakistan killed at least 25 people at an election rally.

The bomber struck as hundreds of people gathered for a meeting of the Awami National Party (ANP), a small, nationalist ethnic Pashtun party, in Charsadda town.

Adm Mullen met President Pervez Musharraf and army chief Ashfaq Kayani on Saturday. The US official said his discussions focused on the security situation in the region.

'Certainly the threat is going up. Certainly in my meetings today, all the leadership expressed concern about being able to eliminate that threat over time,' he said later in the day.

He ruled out direct US intervention to deal with the Al-Qaeda and Taleban militants, who he said had found safe havens in Pakistan's rugged tribal region.

'I give no credence to the notion that the United States could in any way, shape or form invade or attack Pakistan,' he said.

Islamabad has reacted angrily to suggestions that US forces based in Afghanistan could carry out operations in Pakistan's troubled tribal areas, branded a safe haven for Al-Qaeda militants.

Meanwhile, in the North-West Frontier province where Saturday's bombing occurred, bomb disposal experts said that up to 10kg of explosives mixed with metal pellets were used in the attack.

The bombing has further raised fears for the security of the general election scheduled for Feb 18, which has already been delayed by the assassination of opposition leader Benazir Bhutto at a rally in December.

The ANP announced a three-day mourning period and asked its workers to organise condolence meetings and hoist black flags at their offices, said Mr Mian Iftikhar Hussain, the party's provincial information secretary.

Grief and anguish gripped the area as residents buried the dead.

Residents said there were 24 funeral processions in the area yesterday.

Election rallies have been sparse since Ms Bhutto's death in a suicide bomb-and-gun attack in Rawalpindi on Dec 27, and after the government issued a 'security advisory' for candidates to avoid big gatherings.

ASSOCIATED PRESS, AGENCE FRANCE-PRESSE